The present invention relates generally to the field of microwave generating devices, and more particularly to gyrotron oscillators.
The gyrotron is a new type of microwave device employing the electron cyclotron maser mechanism. It ideally comprises an ensemble of almost monoenergetic electrons following helical trajectories around the lines of an axial magnetic field inside a fast wave structure such as a metallic tube or waveguide. The physical mechanism responsible for the radiation in gyrotrons has its origin in a relativistic effect. Initially, the phases of the electrons in their cyclotron orbits are random, but phase bunching can occur because of the dependence of electron cyclotron frequency on the relativistic electron mass. Those electrons that lose energy to the wave become lighter, rotate faster, and, hence accumulate phase lead. Those electrons that gain energy from the wave become heavier, rotate slower, and accumulate phase lag. This can result in phase bunching such that the electrons radiate coherently. Energy transferred from the electrons to the wave is optimized when .omega.-k.sub.z v.sub.zO -s.OMEGA..sub.c .gtoreq.0, where .omega., k.sub.z,v.sub.zo, s, and .OMEGA.c are, respectively, the wave frequency, axial wave number, axial electron velocity, cyclotron harmonic number, and electron cyclotron frequency. In essence, the gyrotron mechanism involves the interaction of a fast waveguide electromagnetic mode and the fast cyclotron wave from an electron beam. These two modes are governed by well known dispersion relationships.
A gyrotron oscillator typically includes a near right circular cavity, in which the interaction of an electron beam with a cavity mode takes place. (The cavity may have some profiling to enhance the efficiency and/or power, but this consists generally of only slight tapering.)
In order to make the gyrotron suitable as a source of high power millimeter wave radiation for such applications as electron cyclotron resonance heating of plasmas and as the current drive in controlled thermal nuclear fusion devices, it is desired to significantly increase the output power of the gyrotron. Increasing the gyrotron power generally requires a higher energy electron beam typically with a larger beam diameter. However, the size of the electron beam typically is limited by the diameter of the interaction cavity. This is particularly so in gyrotrons designed to operate in the fundamental mode. Moreover, RF energy generates current in the walls of the gyrotron interaction cavity. Since these walls are resistive, Joule heating results in the walls. Thus, an increase in the RF power in the gyrotron significantly increases the heating of the cavity walls. In order to obviate these problems, higher order mode cavities are required with larger surface areas in order to reduce the Watts/per square centimeter in the cavity walls and to provide the required volume for larger diameter electron beams. However, with higher mode cavities, the density of modes can prohibit the selection of a single, specified mode with optimum operating parameters, thereby causing decreased efficiency and power.
One method of achieving mode stabilization is to use two separate cavities. This configuration has been termed a gyroklystron and is described in the article "Analysis of A Two-Cavity Gyroklystron," by A. K. Ganguly and K. R. Chu, International Journal of Electronics 51, 503 (1981). In essence, this design adapts the cyclotron maser interaction to a klystron tube. The design includes two separate cavities with the first cavity designed for a low order (TE.sub.011) mode and with the second cavity designed for a higher order (TE.sub.041) mode. The first cavity is a pre-bunching cavity for bunching the electrons in phase space such that all of the electrons are in the right phase so that when they interact with a stronger RF field there will be good energy extraction. The second cavity is the energy extraction cavity. The first and second cavities in this gyroklystron are coupled by the electron beam with a drift region separating the two cavities. It has been found that the pre-bunching mechanism in the first cavity gives the beam a predisposition to interact with a particular frequency. Thus, it has been found that this mechanism can be used to suppress certain modes in the output cavity. In particular, in the case of a TE.sub.011 mode in the pre-bunching cavity, it has been found that the TE.sub.241 mode is suppressed. The above noted article emphasizes the importance of the drift region between the two cavities in order to avoid direct coupling of the RF fields of the two cavities as well as to produce additional phase bunching of the electrons. In essence, the drift region is used to obtain ballistic bunching of the electron beam thereby providing the device with its klystron operating features. However, this drift space causes the gyroklystron to be sensitive to the spread in the beam electron velocities. In addition, the drift space must have a small radial dimension, making interception of the beam with attendant device heating more likely. Thus, the drift space causes the efficiency of the device to be susceptible to changes in the beam velocity spread and to potential space charge effects.